Snapback in a semiconductor device occurs when a drain voltage of a saturated metal oxide semiconductor field transistor (MOSFET) increases over a determined level, and, as a result, drain current increases rapidly. A snapback voltage is a breakdown voltage when a channel is formed between a drain and a source. Among electrons and holes generated by a horizontal field of a channel direction, a hole is ejected to a substrate to lower a junction barrier of the source and the substrate, which may result in the snapback. Specifically, when a drain voltage of a saturated MOST ET increases, pinchoff becomes greater to make a depletion region wider at a drain region. Electrons passing the depletion region gain a considerable amount of kinetic energy from an electric field to turn into hot carriers. The hot carriers collide against the lattice of a covalently bonded substrate to form electrons and holes. At this point, substrate leakage current is generated while the holes travel toward the substrate. Due to the leakage current, voltage drop occurs at the substrate to create a forward bias at a PN junction between the source and the substrate. The forward bias allows electrons of the source to be easily ejected to the substrate. The voltage drop may be proportional to a substrate resistance Rsub. The electron ejected from the source gains energy to form an electron-hole pair while traveling toward the drain. Leakage current flowing to the substrate is also caused by the electron-hole pair, resulting in positive feedback. The above-described mechanism is identical to that of a bipolar junction transistor (BJT). Thus, in the case of an N channel MOS (NMOS) FET, an N-type source, a P-type substrate, and an N-type drain correspond to an emitter, a base, and a collector, respectively, of a BJT.